Linear beam devices are employed in order to amplify signals at high frequencies by modulating an electron beam.
Examples of such devices are klystrons and Inductive Output Tubes (IOTs). Such devices are typically employed as the final stage of amplification in television transmitters at frequencies within the UHF range (470 to 800 MHz). A typical linear beam device comprises an electron gun for generating a beam of electrons, an RF interaction region, for example a series of drift tubes, where amplification of an RF signal takes place and a collector for dissipating the electron beam after it has left the RF interaction region. Amplification of the signal takes place within a vacuum envelope.
Such devices may be of the so-called external cavity type, in which the vacuum envelope comprises a plurality of ceramic cylinders attached to metal structures, for example mounting plates provided on drift tube assemblies.
A problem that may be encountered with such devices is that changes in temperature in the device can give rise to mechanical stress between components of the tube.
It has been proposed to alleviate such problems by the inclusion of so-called balance rings, usually of ceramic, which reduce such thermal stresses. However, it has been found that, in certain conditions, even with the inclusion of balance rings, thermal stresses may be significant and may even cause damage to the ceramic walls defining the vacuum envelope.